In current wireless communication systems such as LTE (Long Term Evolution) and HSPA (High Speed Packet Access), multi-antenna systems are used to increase capacity, coverage, and link reliability. At the base station, antenna arrays are used to create two types of beams.
Type one relates to beams for sector coverage where control and system information are transmitted, e.g., BCH (broadcast channel) and CRS (cell-specific reference signal) in LTE. Since these signals need to reach all users in a cell, they have to be transmitted with a sufficiently wide beam that covers the desired area. The beam should also be sufficiently narrow in order not to transmit too much interference into neighboring sectors. Typically, a beam with 65° half-power beamwidth (HPBW) is used for 3-sector sites, since this provides a good balance between the two conflicting requirements mentioned previously.
Type two relates to beams for user-specific data transmission, e.g., PDSCH (physical downlink shared channel) in LTE. These beams should be narrow in order to maximize the gain to the intended user and also to minimize the interference transmitted to other users.
With a traditional base station antenna, sector coverage is typically provided by a column of radiating elements connected via a feed network to a physical antenna port. The azimuth radiating pattern of the sector-covering beam is in this case given by the individual radiating element. Several such columns can then be assembled adjacent to each other to form an antenna array in the horizontal dimension. By applying beamforming weights to this array, user-specific beams can be created. In LTE, several transmission modes have been specified that make use of user-specific beamforming. One example is transmission mode 4 (TM4) where beamforming weights are selected from a set of predefined weights in a codebook, so called codebook-based precoding.
Active antenna arrays may also be used, in which each radiating element, or a group of radiating elements, is equipped with its own radio branch. With active antenna arrays, the generation of sector-covering beams becomes more flexible since these beams can be created from several radiating elements by means of beamforming. A sector-covering beam is then associated with a so called virtual antenna port. User-specific beamforming is performed by applying weights to a plurality of such virtual antenna ports.
The flexibility in the sector beam generation can be utilized for sector shape reconfiguration when changes occur in the network such as changes in deployment or spatial traffic distribution, e.g., new sites, buildings, or traffic hotspots. It is well known that such reconfiguration can give substantial improvements in system performance.
An effective means for sector shape reconfiguration is to change the azimuth beamwidth of the sector covering beam, thereby changing the width of the sector. A resulting problem with this is that the beams in the predefined codebook are not suited for the new sector width. Both the beamwidth and pointing directions of these beams may be poorly matched to the sector width. For example, a codebook beam pointing inside the sector before reconfiguration may point outside the sector after reconfiguration. With the original sector beam pattern, before reconfiguration, all user-specific beams point within the sector. After a reconfiguration that has made the sector narrower, some of the user-specific beams point outside the new narrow sector.
This means that when changing the sector width in wireless cellular networks by means of reconfigurable antennas, the angular coverage of the precoder beams in a predetermined codebook (such as in LTE) may become poorly matched to the new sector width. This can result in high interference and reduced coverage of precoded user data.
It is therefore a desire to provide a node in a wireless communication system that comprises an antenna arrangement that enables changing of the sector width in wireless cellular networks, where all beams are matched to the new sector width.